In the prior art it is known that multiple layer printed circuit boards have been used as a means for the support of and electrical interconnection to a plurality of magnetizable memory cores--see the J. Y. Huie, et al, U.S. Pat. No. 3,626,586--or for the routing of complex electrical circuit paths including signal, ground and voltage planes--see the K. J. Varker U.S. Pat. No. 4,030,190. Such multilayer printed circuit boards include patterns of "via" holes and "through" holes for the inter-layer inter-connection of the circuit paths on the several printed circuit boards--see the W. E. Dougherty, Jr. U.S. Pat. No. 3,739,469.
Epoxy resin/glass laminates of an epoxy-glass layer having conductive layers affixed thereto are formed into multilayer printed circuit blards using laminating layers of epoxy prepeg under appropriate pressure and temperature. See the publications "Design and Fabrication of Multilayer Printed Circuit Boards", R. G. Greygoose, et al, Solid State Technology, November 1970, pp. 33-40, and "Double-Multilayer Printed Wiring Boards. A Novel Technique to Overcome a Packing Density Problem", B. R. Smith, Internepcon, 1976, pp. 41-45. However, such epoxy resin/glass multi-layer printed circuit boards have low heat resistance, poor dimensional stability, high coefficient of thermal expansion and a less than desirable dielectric constant at high frequencies. These deficiencies, however, are acceptable in many applications and are compensated for by relatively low material cost and straight-forward and inexpensive fabrication methods.
However, in applications requiring very high current densities, the deficiencies of epoxy resin/glass multilayer printed circuit boards are not acceptable. In such applications, the computer industry has employed multilayer printed circuit boards formed of polyimide resin/glass laminates. See the publications "The Development of Polyimide Multilayer Boards Containing Integral Flexible Circuitry", J. R. Cannizzaro, Internepcon 1970, pp. I-70-40, and "Multilayer Printed Wiring Boards for Demanding Environmental Applications", W. S. Rigling, Internepcon Europa, 1973, pp. 6-12. Such polyimide resin/glass multilayer printed circuit boards have high heat resistance, good dimensional stability, low coefficient of thermal expansion and desirable dielectric constant; however, such desirable characteristics are accompanied by relatively high material cost and expensive fabrication methods.
In the J. R. Cannizzaro, et al, U.S. Pat. No. 3,760,091 there is taught the fabrication of multilayer printed circuit boards formed of layers of substantially hydrophilic material, e.g., epoxy-glass, having conductive layers, e.g., copper, therebetween. Layers of substantially hydrophobic material e.g., polyimide-glass, are sandwiched therebetween. Through-holes are electroless copper plated with the copper adhering to all layers except the polyimide layers forming a series of via holes along the through-holes forming independent connections between the sandwiching polyimide layers. In this configuration the bonding layers, used to bond the glass-epoxy layers, the conductive layers and the polyimide layers, consist of layers of B stage prepreg containing an epoxy resin.